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species is limited by slow growth, shallow root system, and small biomass produc-
tion. In addition, the plant biomass must also be harvested and disposed of properly,
complying with RCRA standards (Emerging Technologies for the
Phytoremediation of Metals in Soil 1997).
There are several factors limiting the extent of metal phytoextraction including:
• Metal bioavailability within the rhizosphere
• Rate of metal uptake by roots
• Proportion of metal “fixed” within the roots
• Rate of xylem loading/translocation to shoots
• Cellular tolerance to toxic metals
In order for this cleanup method to be feasible, the plants must extract large
concentrations of heavy metals into their roots (Blaylock and Huang 1999), trans-
locate the heavy metal into the surface biomass, and produce a large quantity of
plant biomass (Brennan and Shelley 1999). In addition, the plants must have
mechanisms to detoxify and/or tolerate high metal concentrations accumulated in
their shoots (Brennan and Shelley 1999).

4.4.2 Phytostabilization

This is also referred to as in-place inactivation and is primarily used for the
remediation of soil, sediment, and sludge (United States Environmental Protection
Agency 2000). It is the use of plant roots to limit contaminant mobility and
bioavailability in the soil (Emerging Technologies for the Phytoremediation of
Metals in Soil 1997). The plants’ primary purposes are to decrease the amount of
water percolating through the soil matrix, which may result in the formation of a
hazardous leachate (Blaylock and Huang 1999), act as a barrier to prevent direct
contact with the contaminated soil, and prevent soil erosion and the distribution of
the toxic metal to other areas (Raskin and Ensley 2000). Phytostabilization can
occur through the sorption, precipitation, complexation, or metal valence reduction.
It is useful for the treatment of lead (Pb) as well as arsenic (As), cadmium (Cd),
chromium (Cr), copper (Cu), and zinc (Zn) (United States Environmental Protec-
tion Agency 2000). Some of the advantages associated with this technology are that
the disposal of hazardous material/biomass is not required (United States Environ-
mental Protection Agency 2000), and it is very effective when rapid immobilization
is needed to preserve ground and surface waters. The presence of plants also
reduces soil erosion and decreases the amount of water available in the system
(United States Environmental Protection Agency 2000). However, this cleanup
technology has several major disadvantages including contaminant remaining in
soil, application of extensive fertilization or soil amendments, mandatory monitor-
ing is required, and the stabilization of the contaminants may be primarily due to the
soil amendments (United States Environmental Protection Agency 2000).
Phytostabilization has been used to treat contaminated land areas affected by